Power network protector relays are used to monitor and control the power flow of low-voltage AC to secondary network power systems. They are widely used in underground distribution networks with multiple power injection points that require high continuity of service in heavily loaded, high-population-density urban areas. The purpose of the network protector is to prevent the system from backfeeding and initiating automatic reclosing when the system returns to normal. This paper describes the operation theory, application areas and testing procedures of network protectors. The test results of a microcomputer-based network protector are used in this paper for illustration.